Racemization of amino acid



Patented Feb. 19, 1952 UNITED STATES PATENT OFFICE RACEMIZATION or AMINO ACID Robert D. Emmick, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours. and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 29, 1950, Serial No. 171,245

8 Claims. (Cl. 260-534) essential component of animal diets whereas the 10:

enantiomorphic d() -1ysine has no known nutritional value.

Lysine which has been synthesized from optically inactive materials, for example, by the process of Eck and Marvel, Organic Syntheses Collecl5,

tive vol. II, page 374, or by the process described in U. S. P. 2,498,300 is optically inactive and consists of equal parts of the biologically active l(+) isomer and the biologically inactive d() -isomer.

Thus, its nutritional value can be doubled by con verting its d()-lysine content into the l'('+) form while retaining the quantity of the latter initially present.

It is possible to convert synthetic lysine completely to the biologically active form by first 5 separating the isomers (resolution) and then converting the inactive d()-lysine back to the dl-mixture (racemization). By a repetition of these operations, the lysine is eventually converted entirely into the l(+)-form. As a practical matter it is unnecessary and usually impossible to effect a complete separation of the l'(+) -lysine and the d()-isomer. The requirements for an economically feasible process are that a substantial portion of the l(+) -lysine be isolated in pure form in the resolution step and that no serious loss of total lysine occur in either step.

Resolution of dl-lysine is usually effected by combining the material to be resolved with an optically active compound known as the resolving agent and fractionally crystallizing the product. This may be accomplished by known methods. For example, in a method disclosed by C. P. Berg in the Journal of Biological Chemistry, vol.

115, pages 9-15, (1936), d-camphoric acid is used Y as a resolving agent to obtain l(+) -lysine. This author also shows the use of l-camphoric acid to obtain d() -lysine The racemization of lysine, or conversion of the optically active material to the racemic mix- I ture, has previously been accomplished by heating the isomer with hydrochloric" acid. This method is disclosed in Ber. 35, 3778, (1902). Although good recoveries of lysine are obtainableby this method, the mixture of hydrochloric acid 66 The lysine 5 racemization at a practical rate. sive corrosion resistant equipment is necessary if this method is to be used. Furthermore, the use of hydrochloric acid requires equipment capable of withstanding elevated pressures. Attempts to use this method by reducing the proportion of hydrochloric acid to that stoichiometrically equivalent to the lysine present have failed to obviate this serious disadvantage. Another method for the racemization of lysine which is disclosed in U. S. P. 2,071,327 consists in heating the aminoacid with acetyl chloride in the presence of acetic acid. Obviously, this method is even less desir able than the one described above.

It is an object of this invention to provide a simple, practical and economical process for the racemization of lysine. A further object is to provide a new and improved process by which a mixture of l(+) and d() -isomers of lysine may easily be recovered from the reaction medium.

Still another object is to provide a rapid process for the, racemization of lysine which may be operated at atmospheric pressure. These and other'objects will be apparent from the following description of the invention.

The above objects are attained in accordance with this invention by heating optically active lysine in combination with phosphoric acid.

In one mode of operating the process of this invention phosphoric acid is added to an aqueous solution of optically active lysine or lysine hydrochloride and the resulting solution is "boiled to remove water until the desired temperature of operation is reached. A reflux condenser is then attached and heating is continued until racemization is substantially complete. The racemized lysine may be recovered by diluting the racemization mixture and passing the resulting solution into contact with a cation-exchange material which adsorbs the lysine. 'Lysine may then be displaced from the cation-exchange material by elution with a base, for example, ammonia, and free lysine obtained by evaporation to remove the ammonia. An acid, for example, hydrochloric acid, may be utilized if desired to remove the lysine from the cation-exchange material in which case thelysine may be recovered as the hydrochloride.

Racemization in accordance with this invention may be carried out by heating either free lysine or lysine monohydrochloride in combination with phosphoric acid. When the racemization is carried out using the hydrochloride a somewhat longer time is required but better recovery of lysine is obtained and higher yields of optically active lysine are obtained in the subsequent resolution step.

The time required for complete racemization 5 will vary with the temperature as well as the ratio of optical isomers :-present in 'therlysine" treated. For'example, complete'racemizat-ion of l(+)-lysine has been obtained by heating with sorbs the lysine and allows the phosphoric acid to phosphoric acid during 1 hour at about 170 td lo material for eluting the cation-exchange material 175? C. Lysine containing about:'70%.-of=--thet d(-) -isomer was completely racemized by heating with phosphoric acid during l hour-M 155 to 165 C. Racemization of -l(+)-'-lysine-mono-'- hydrochloride required 4 hoursat 160"'to162*C." 1'5" tures the racemization is "slower while at higher temperatures?although the ratepf racemization is incrjeased j si'dereactionsand. decompositioni of lysi'rie maypccur *vv-ith 're'sultii'ig =low recoveries of lysine-5 Although rac'emizationmay'be'carried to com U.'S5-P;'2,366,007.1 Otheri-examplesiinclude p'oly u. phenolsulfonic acids, sulfonated hydrocarbon" Sulfonated coals and natural. and'..synthetic zeolites: having:

pletion-in accordance =withthis invention it is unnecessary =and may-sometiines' be undesirable in-- practical -operation'=- to-"racemize completely:

Sincemixtures containing l varying proportions of"l(-"+ )-lysine and"'d ly'sine may' be -resoIved": L the practice of this invention. The selection of a particular: cation-exchange --ma-terialz'willt. depend upon such considerations as capacity., exchange without difiiculty it is entirely s atisf actory to stop-i: the racemization at 'anydesired point. However; it is-of course" desirable iii-increasesubstantially the proportion ofithedesirediisomer inatlieimixe tare-5 Although lysine mavbe racemiz'eziby heatinginf combination-with-phosphoric acid in the -absence of-water it* is preferable to' move present-at least Fr 2% by weight 'of wat'er-based upon the'wei'ghtof solubleipolymeric materials which-contain acidic groups capable of combining 'i withv a :variety: of cations;tolfo'rm insoluble 'saits::. A typical example of" a catiom exchange;- material is: describe'dz' in polymersrand': polycarboxylic" acids:

cation-exchange: properties. are zal'so suitable :in

rateymechanical ruggedness andz'ch'emicalz stabilitysi'as :welluasxcost; Any. off':tlie.'.icommercially' availableicatiorr exchangelmaterials are suitable. Fortexample;z-satisfactoryi results may be; obtained '3 with catiomexchangeirmaterials sold. under the:

the mixture in order tomir'iimize 'del'iydration:of"40 :Di1olite (II-3;." "Amberlite:i[RC-'-50 and: Permu:-.

the lysine:-- The racernization may also -b"'eaccom= plish'ed in the presence of large" amounted water;:- for example: *7i by weight; but sucli large amounts of water will involve a longer" time: for

racemization because-of tn'mowertemperatureor" 5 drocarbonipolymerszx These cation exchange-ma+ terials areiparificularly'valuable; in the practicaof 2 this? inventiohtsincwthey 'possessiexcell'ent .chemi'-' cal and Lphysical stability -Iunder? the conditions:

operation or pressureequipment -in' ordr 'to ob tain the preferred temperatur'e' of operatibn. Ifi general it i's desirable to adjizstiitlie amount of water utili'zed -in accordance with 'tlie-temperm A'rpreferred class-of cationteexchangeimaterials" are those in which the acidic glOllpSSaIBlSlflfOniC."

aci'd igroups asexemplified bythe; sulfonated: hy-

utilized; arelreadilyravailabl,"and have highrcature at wliich -tlie operation to be carriedt: out-. 501 pacit'y: and exchange rate.-

phosphoric acid to rmole of lysines hascbeen used withfexcelInt results A greater? excess mav be utiliaed if' des'iredbut n' inost' cases-no advan ta'ge' is bbtairied th'ereb :3"

when thefracemizatibflih&befiical'riditd the: uo teri'aitistused kthet cationi exchange material must be-.-la"st1tong:acid:.:type; for :example,=- a: material in'" WhiC-BI'ITTthETaOidlJCI group: are sulfonic' 'acidfgroups.

Th'us th cationtezexchanget material; whenzusedin desired point -tHe Iysine :may" bei'sepa1:ated-ircrnii the phos horic acid-Tand ther'latter 'iS-ZSllil78.b18ffdf"' reuseintheraeemiiationiorocessiz variou'soneth ods maysbe utilized rfor::theiseparationi-oflysine Various :formstofthe zcation'i ex'change material. maysbe' utili'Zedbut :the acid form or ammonium formarezmost suitablerand the acid form 'is'usual'e necessity ':of fconvertingzthe ammonium; or other form to the free acid for reuse in' the racemization Step;

when: the? acid: form of aication exchan'ge-mathe'acidzformshould 'be-suiiiciently: strong to libfrom:thenhosphoricracida: For example: a'-'conr-'- ermimphosphoflc c d The following examples villustratethe invention;

Ezcample' 1' A soluti'on of 12.80' g.- (0.07 mole) of -l -1yslne droxides are examples of materials"suitable-"for Emon hydfgchloride--a,nd "10 35% phosphoric A preferredemethodiofrisolatingt thezlysirre fol'a lowi'r'i'g racemizatibn' ospassitneiunutedfrace mi'zation '-m-iictiire into contact with a :cadJon acid in 3U mlrwater was boileduntil the temperatu're reached 185 0. and! 'was then 'heated under reflux at"-185 to'192'C. for L hour. The solution;- & v after diliition with water, showed zero rotationexchange ma'terial in tlie' acid form whiclf ad indicatingi completeracemization. The-solution" was diluted to 1 liter and the lysine adsorbed on a column of cation-exchange resin. The resin was C1- (theory 32.4) and a water solution showed zero rotation. Example 2 To a water solution containing 0.07 mole free l(+)-lysine was added ml. 85% phosphoric acid. The mixture was heated until the temperature reached 170 C. and was then heated under reflux at 170-175 C. for 1 hour. The mixture;

after dilution with water, showed zero rotation.

boiled until the temperature reached 160 C. The

solution was then heated under reflux at 160 C. for 1 hour, diluted with water, and boiled minutes. It was found to be completely racemized.

2 Free lysine was recovered by adsorbing the lysine.

on a cation-exchange resin and eluting it with ammonia; the recovery'was 92.5 percent. Reso-.

lution of the lysine with d-camphoric 'acid in aqueous methanol gave a 40 percent yield 01' The lysine dihydrochloride, recovered as described in Example 1, weighed 14.44 g., 94 percent 0! the theoretical.

: Example 3 Toa water solution of 0.14 mole free lysine con-j I taining about 70% d( -isomer, was added 20 ml.

85% phosphoric acid. The mixture was boiled until the temperature reached 155 C. and was then heated under reflux at 155-165 C. for 1 hour.

The mixture, after dilution with water, showed zero rotation; The lysine dihydrochloride recov ered as described in Example 1, weighed 30.64 g.,

99.9 percent of the theoretical.

Example 4 To 68 g. of l(+) -lysine monohydrochloride was added 53.5 ml. 85% phosphoric acid and about 20 ml. water. The mixtures was boiled until the temperature reached 160 C. A reflux condenser was then attached and the mixture heated at 160 to 162 C. for 4 hours. Samples (5 ml.) were taken atvarious intervals, diluted to 25 ml., and the rotation observed. The results are summarized in the table below:

Time of Heating Observed Rom" at 160 to 162 C.

(Hmus) tion (Degrees) Example 5 The sameprocedure as in Example 4 was used. The racemization was run at 150 to 153 C. The results are summarized in the table below.

Observed Rota- Time of Heating 53 tion (Degrees) at 150-1 C.

Example 6 A water solution of 1.055 mole free lysine co'ntaining-about 70% d()-isomer was mixed with 143 ml. 85% phosphoric acid and the solution l(+)-lysine d-camphorate; 94% optically pure.

The methanol was stripped from the mother liquor which was then treated with 114 ml. phosphoric acid. The theoretical quantity of .d-camphoric acid was recovered by filtration and extraction with ether. The racemization and reconversion to free lysine was repeated with an' 89 percent recovery of lysine. This was resolved with d-camphoric acid and gave a 43 percent yield of optically pure -l(+)-lysine d-camphorate.

The resolution mother'liquor was again converted to lysine phosphate, racemized and reconverted tofree lysine. The recovery of lysinewas 87 percent.

Example 7 A series of runs similar to Example 6 was made in which lysine monohydrochloride was treated j with phosphoric acid and racemized at C.

The results are shown in thetable below.

Per Cent l(+)-lysine d-Oamphorate Recovery Free Lysine Racemization Per Cent cal Purity In the first run 97.5% optically pure l(+)-lysine monohydrochloride was used as the starting material.

The process of this invention is superior to the prior art process utilizing hydrochloric acid in the surprising and unexpected speed with which racemization is accomplished. It has been found that racemization of lysine with hydrochloric acid at 185 C. requires about 12 hours as compared with only about 1 hour for the process of this invention at a temperature of to C. Thus the racemization with phosphoric acid may be carried out at a much lower temperature and still require only a fraction of the time required by the prior art method. In addition racemiza-, tion with phosphoric acid may be carried out at atmospheric pressure.

Although the process of this invention is useful in racemizing optically active lysine from any source, it is of particular utility as a part of the resolution-racemization cycle involved in converting dl-lysine or, other mixture of l(+) and d()-lysine to a pure optical isomer. This procedure which may vary in detail consists essentially in resolving lysine by known methods, for example, the method described by Berg, separating the desired isomer," heating the remaining undesired isomer in combination with phosphoric acid to effect racemization, recovering a mixture of l(+) and d() -lysine and returning the latter to the resolution step.

When the lysine to be racemized in accordance with this invention is in the form of its compound with a resolving agent such as optically active camphoric acid, the latter may be recovered from solution upon the addition of phosphoric acid followed by filtration and extraction with ether and racemization is then carried out 7' by heating the-remaining solution of lysine in combination with? phosphoric :acid. "as described above.

'Throughout'this-specification and. in the appended claims the term lysine, unless :otherwise indicated, is'intended to include'within itsrscope bothufreelysineand alysine hydrochloride orrother salt oil-lysine.

I claim:

:1. Process for the racemization 'of'lysine-which comprises heating optically active lysineiin combination with phosphoric'acid.

2. Processfcr the racemiza'tion of lysine which comprisesxheating optically active lysine in combination with'p'hosphoric acidin the presence of water.

3. "The process of claim 2 wherein'the'temperaturetismaintainedat 11 0" to210 C.

:4. The process-of claim 2 wherein the concentration of phosphoric acid is at least equimolar;

15'; .Process which:comprises: resolving a mixture 0f-"'1(+)and'd( ).-lysine, separating a desired optically active isomer, heating the remaining undesired optically active isomer in combination with phosphoric acid and recovering a mixture of l(-|--) and d(- )--.lysine.

6. .Process which comprises resolving 'a .mixture of 1'(+) and d'( )--lysine into 1(+) -lysine and d()-lysine', separating :l(-|-.).-lysine, heating the remaining d( -lysine in combination with ph0sphoric acid in thepresenceioi'water andrecovering a mixture of l(+) and'd,(-) -lysine.

7. Process which comprises heating; optically active lysinein'combination with phosphoric'a'cidin the presence of water, passing an aqueous solutionof the racemized lysine obtained thereby into contact with'a cation-exchange materialgrecovering phosphoric acid, eluting said cation-exchange material with a material capable of displacing lysine from said cation-exchange material and recovering lysine, from the elutriate.

8. Process which comprises heating optically active lysine in combination with phosphoric acid,

in the presence of water, passing an aqueous solution of the racemized lysine obtained'thereby into contact with a cation-exchange material, re-

REFERENCES CITED The following references are of record in the file of this patent:

Neuberger: "Advances in Protein Chemistry,.

vol. 4, edited by Anson et al. v(Academic Press) p. 339 (1948.).

Block et. al.: The Amino Acid Composition 01. Proteins and Foods, .(Thomas) -.pp. 292-293 

1. PROCESS FOR THE RACEMIZATION OF LYSINE WHICH COMPRISES HEATING OPTICALLY ACTIVE LYSINE IN COMBINATION WITH PHOSPHORIC ACID. 